1. Field of the Invention
The present invention relates to a novel process for methanol production. Methanol is an inexpensive and useful compound having a wide range of applications as an intermediate material for a variety of chemicals and as a solvent, a fuel for vehicles or a thermal power plant by itself.
2. Description of the Related Art
The process using carbon monoxide and hydrogen, the so-called vapor-phase method, had been industrially used for many years, for methanol production. For example, in 1913, BASF Inc., a German company, suggested that it may be possible to produce oxygen-containing compounds including methanol, from water gas at a temperature of at least 300.degree. C. and a pressure of at least 100 atm., using a catalyst mainly containing an oxide of, e.g., Cr or Zn. Subsequently, the so-called high pressure methanol method had been initiated in many countries. In 1959, ICI Inc., a British company, setting in a high level of desulfurization technique of a synthesis gas, developed the so-called low pressure methanol method, where the reaction is conducted using a catalyst mainly containing CuO, at a lower temperature and a lower pressure than the previous method, i.e., at 200.degree. to 300.degree. C. and 50 to 150 atm. Then, there have been many improvements in catalysts and processes. Currently, most of processes of methanol production adapt a low pressure method at a reaction temperature of 200.degree. C. and a reaction pressure of about 100 atm. In such a vapor-phase, low-temperature and low-pressure method, copper chromite has been used. It has been described that a Raney copper-zinc catalyst may be utilized as a copper catalyst for methanol production in a vapor phase. For example, J. Catal.,. 80, 1-13(1983) and J. Catal., 80, 14-24(1983) have disclosed vapor-phase processes for methanol synthesis from carbon monoxide or carbon dioxide and hydrogen, using a Raney copper-zinc catalyst which can be prepared by developing a mother alloy containing 50 wt. % of aluminum, 30 to 36 wt. % of copper and 14 to 20 wt. % of zinc. They have described that a Raney copper catalyst containing zinc may have increased specific surface area, resulting in its improved activity.
Furthermore, Japanese Patent Laid-Open No. 197110/1995 has disclosed a continuous process for production of methanol, using a Raney copper alloy with a controlled particle size, prepared by adding and dispersing, on a disk rotating with a high speed, a melt of copper alloy consisting of 30 to 60 wt. % of copper, 0.5 to 25 wt. % of zinc and 0.5 to 10 wt. % of chromium or aluminum, and has described that its physical properties such as size distribution may not be changed even after 120 hours of the reaction.
There has been, however, a problem in these vapor-phase processes for methanol production that due to some problems in the process such as heat removal, it is difficult to scale up the process for inexpensively producing methanol in a large amount.
The reaction for methanol synthesis from carbon monoxide and hydrogen is exothermic as shown in the following equation and is an equilibrium reaction. EQU CO+2H.sub.2 .fwdarw.CH.sub.3 OH .DELTA.H.sub.298 =-21.7 kcal/mol
A lower temperature and a higher pressure may be, therefore, favorable for methanol synthesis. A highly active catalyst at a lower temperature may be favorable because it may markedly improve the conversion rate of the raw material gases, eliminating necessity for recycling unreacted gases into the reaction system. Industrially, a catalyst highly active in methanol production at a lower pressure than that in a synthesis gas production, is extremely favorable because it can eliminate necessity of pressure elevation during introducing the synthesis gas into a reactor for methanol production. Recently, a low-temperature liquid-phase methanol synthesis has been of interest because reaction in a liquid phase may be effective in heat removal which is a problem in a vapor-phase method.
The prior art in this low-temperature liquid-phase methanol synthesis will be described.
There are known several catalysts active to some extent at a low temperature and a low pressure, e.g., below 160.degree. C. and below 50 atm. Among others, copper or nickel catalysts are known to be highly active. It is, however, well known that a nickel catalyst may generate nickel carbonyl which is extremely toxic, making it very difficult to handle the catalyst.
For a copper catalyst, Japanese Patent Publication No. 51130/1988 has disclosed a synthesis of oxygen-containing compounds where carbon monoxide and hydrogen are reacted using a sodium or potassium alkoxide and a copper compound except copper oxides as catalysts. This publication has shown specific monovalent and bivalent copper compounds.
Japanese Patent Publication No. 2686/1994 (WO 86/03190) has disclosed a process for methanol production where in the presence of a catalyst consisting of a copper compound prepared according to Adkins method and an alkali metal alkoxide, carbon monoxide and hydrogen are reacted in a liquid phase, using a liquid reaction medium in the reactor comprising methanol and methyl formate as well as at least 50 vol. % of a nonpolar organic solvent having a lower dielectric constant than that of pure methanol at the same temperature. It has been described that the most suitable copper catalyst is copper chromite prepared by preliminary hydrogen reduction of a pyrolysis product of ammonium dichromate.
Appl. Catal., 103, 105-122 (1993) has also disclosed similar copper-chromium catalysts, describing that for methanol synthesis from carbon monoxide and hydrogen, using a copper-chromite catalyst and potassium methoxide (or sodium methoxide) may reduce the reaction temperature by about 100.degree. C., resulting in significant reduction in the amount of the recycle gas.
U.S. Pat. Nos. 4,992,480 and 4,935,395 have disclosed a process for methanol production from a synthesis gas using a homogeneous catalyst consisting of an alkoxide and a carbonyl compound of metal selected from the group consisting of Cu, Ni, Pd, Co, Ru, Mo and Fe.
As far as we have verified it, there have been no catalysts with an adequate activity for low-temperature liquid-phase methanol synthesis. Specifically, a copper catalyst having some valence including copper chromite catalysts such as copper chromite may be subject to reduction and conversion of the copper oxide on the surface into metal copper, resulting in significant deterioration of its activity. Carbonyl compounds have some problems such as difficulty in handling, and thus cannot be used in an industrial scale.
A metal alkoxide is essential for producing methanol at a low temperature and a low pressure. It has been found that a part of the metal alkoxide used is lost by conversion into a formate during the reaction. It has been also found that the converted amount may be increased with increase of the metal alkoxide. Thus, the metal alkoxide should be used in a substantially less amount than that known in the prior art; otherwise, the process may be quite uneconomical.
On the other hand, use of methanol as a solvent may be very advantageous because it can eliminate separation of the product from the solvent. All the catalysts for low-temperature, liquid-phase methanol synthesis have a poor activity in methanol which is also the product. Even when used in another solvent system, these catalysts may be subject to deterioration in their activity due to methanol sequentially produced. Therefore, there has been a strong demand to obtain a catalyst which is little deteriorated at a low temperature and a low pressure in the presence of methanol and is highly active with a low concentration of a metal alkoxide.